1. Field of the Invention
The present invention relates to an erecting equal-magnification lens array plate used in image reading devices and image writing devices.
2. Description of the Related Art
Some image reading devices such as scanners are known to use erecting equal-magnification optical systems. Erecting equal-magnification optics are capable of reducing the size of devices better than reduction optics. In the case of image reading devices, an erecting equal-magnification optical system comprises a linear light source, an erecting equal-magnification lens array, and a linear image sensor.
A rod lens array capable of forming an erect equal-magnification image is used as an erecting equal-magnification lens array in an erecting equal-magnification optical system. Normally, a rod lens array comprises an arrangement of rod lenses in the longitudinal direction (main scanning direction of the image reading device) of the lens array. By increasing the number of rows of rod lenses, the proportion of light transmitted is improved and unevenness in the amount of light transmitted is reduced. Due to cost concerns, it is common to use one or two rows of rod lenses in an array.
Meanwhile, an erecting equal-magnification lens array plate could be formed as a stack of two transparent lens array plates built such that the optical axes of individual convex lenses are aligned, where each transparent lens array plate includes a systematic arrangement of micro-convex lenses on both surfaces of the plate. Since an erecting equal-magnification lens array plate such as this can be formed by, for example, injection molding, an erecting equal-magnification lens array can be manufactured at a relatively low cost.
An erecting equal-magnification lens array plate lacks a wall for beam separation between adjacent lenses. Therefore, there is a problem in that a light beam diagonally incident on an erecting equal-magnification lens array plate travels diagonally inside the plate and enters an adjacent convex lens, creating noise (referred to as ghost noise) as it leaves the plate.
There is known an erecting equal-magnification lens array plate in which a light-shielding member (hereinafter, referred to as an intermediate light-shielding member) is provided between the two lens array plates in order to reduce ghost noise (see, for example, patent document No. 1).
[patent document No. 1] JP2009-069801
The intermediate light-shielding member of the erecting equal-magnification lens array plate disclosed in patent document 1 is sandwiched by two lens array plates. In other words, the lens-to-lens distance of the two lens array plates is determined by the thickness of the intermediate light-shielding member. In the erecting equal-magnification lens array plate, the lens-to-lens distance counts highly for the purpose of achieving desired optical property. Therefore, the thickness of the intermediate light-shielding member should be precisely controlled in the case of the erecting equal-magnification lens array plate disclosed in patent document 1.
The intermediate light-shielding member is formed by, for example, injection molding, using a light absorbing material such as black ABS resin. The intermediate light-shielding member is formed by a pin-shaped mold that forms a through hole and a receiving mold coupled to the pin-shaped mold, in order to provide a plurality of openings (through holes) corresponding to the plurality of lenses of the lens array plates. However, since a pin-shaped mold is used in manufacturing the plate, burrs extending along pins (also called, vertical burrs) are likely to occur from the through hole in a direction that runs perpendicularly to the lens plate. Where the intermediate light-shielding member is sandwiched between two lens array plates as in patent document 1, a vertical burr occurring in a through hole may be sandwiched between the through hole and the lens, creating a gap and adversely affecting the optical property. The problem will be solved shaving the burrs by polishing. Since the burrs are very small, however, it would not be easy to remove the burrs. Even if the burrs can be removed, the manufacturing cost is increased due to the extra step of removing the burrs.